1. Field of the Invention
The present invention is directed generally to optics. More particularly, the present invention relates to equalizing output intensity of a light beam useful in lithographic processing.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus may be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, light is directed to a patterning device, which may be referred to as a mask, a reticle, an array of individually programmable or controllable elements (maskless), or the like. The patterning device may be used to generate a circuit pattern corresponding to an individual layer of an IC, flat panel display, or other device. This pattern may be transferred onto all or part of the substrate (e.g., a glass plate, a wafer, etc.), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. The imaging may include the processing of light through a projection system, which may include optical components such as mirrors, lenses, beam splitters, and the like. Other components or devices may exist in a lithographic apparatus that can also contain optical components.
Some lithography apparatuses require extremely small dose control variation. For example, some tools require less than 0.5% dose control variation for single pulse printing. The pulse to pulse variation from a light source, such as a laser, can be upwards of 10% or more. One currently-used solution to this problem is the use of a Pockel's Cell. A Pockel's Cell is an electro-optic device that can be used as a switch. It modifies the phase between two polarization states (fast and slow axis). A Pockel's Cell can be used between crossed polarizers to modulate, or control, the intensity of an optical beam. The effect on which a Pockel's Cell device is based is called Pockel's effect or linear electro-optical effect. There are many downsides to using a Pockel's Cell approach, however. These downsides include the requirement of a very high voltage, optics that take up a large volume of space and are very complicated, extremely complex detection and switching electronics, and deterioration of transmission due to two photon absorption/color center formation.